Limited current circuit for electro-luminescent lamp inverter

ABSTRACT

A current limiting device includes a constant current source circuit constructed and arranged to be connected to a direct current input voltage source and to regulate electrical current and a resonance source circuit receiving current from the constant current source circuit. The resonance source circuit is constructed and arranged to be electrically connected with an electro-luminescent (EL) lamp so that the EL lamp receives alternating current. The constant current source circuit and the resonance source circuit are constructed and arranged such that when current flows from the constant current source circuit to the resonance source circuit and to the EL lamp, a maximum peak current received by a user contacting an output of the device or contacting the EL lamp is less than about 0.7 mA.

This application claims the benefit of the earlier filing date of U.S.Provisional Application No. 60/787,534, filed on Mar. 31, 2006 which isincorporated by reference herein in its entirety.

FIELD OF THE INVENTION

The invention relates to electro-luminescent (EL) lamps and, moreparticularly, to a circuit for limiting current of an EL inverter toprevent or reduce electrical shock to a user.

BACKGROUND OF THE INVENTION

The use of electro-luminescent (EL) panels, particularly in portableelectronic derives such as keyboards of laptop and desktop computers,requires meeting various standard safety tests. In order to produce ahigh level of backlighting, a high peak to peak voltage is necessary. Toavoid electrical shock, organizations like UL (UnderwritersLaboratories), require that the inverter, used to power the EL panel(s),provides the power necessary such that the clearance and creepagedistance between the high voltage output of the EL panel and itssurroundings (enclosure, printed circuit board and connector) do notcreate a hazardous condition (UL 60950). The requirement of a safetyspace usually needs 4-8 mm. For portable and lightweight devices wherespace is at a premium, it is difficult to meet such standards. Thealternative method to meet such standard is to provide limited currentcircuit (LCC) in the inverter. The equivalent impedance of a human bodyis about 2000 ohms. For a 2000 ohm load between outputs or output toground, a maximum peak current flow should not exceed 0.7 mA.

Conventional single ended autotransformer type inverters inhibit theoscillation as the output load is shorted, but do not take a 2000 ohmload into account. Such an inverter would protect itself from an outputshort circuit, but would not prevent an electrical shock to the humanbody upon contact with the output of the inverter under normalconditions.

Thus, there is a need to provide an inverter circuit with capability tolimit current such that a maximum peak current flow to ground does notexceed 0.7 mA.

SUMMARY OF THE INVENTION

An object of the invention is to fulfill the need referred to above. Inaccordance with the principles of the present invention, this objectiveis obtained by providing a limited current circuit (LCC) within aninverter of an EL lamp to prevent or limit the above-mentionedelectrical hazard. Thus, the LCC is configured so that when a human bodyor a similar resistance (2000 ohm) touches the output of the inverter ortouches the EL lamp under normal conditions, there is a maximum peakcurrent flow to ground. Present UL requirements specify this at notexceeding 0.7 mA.

Thus, a current limiting device is provided that includes current sourcecircuit constructed and arranged to be connected to a direct currentinput voltage source and to regulate electrical current and a resonancesource circuit receiving current from the current source circuit. Theresonance source circuit is constructed and arranged to be electricallyconnected with an electro-luminescent (EL) lamp so that the EL lampreceives alternating current. The current source circuit and theresonance source circuit are constructed and arranged such that whencurrent flows from the current source circuit to the resonance sourcecircuit and to the EL lamp, a maximum peak current received by a usercontacting an output of the device or contacting the EL lamp is lessthan about 0.7 mA.

In accordance with another aspect of the invention, a method of limitingcurrent of an inverter associated with an electro-luminescent (EL) lampis provided. The method provides an inverter constructed and arranged toconvert Direct Current input voltage to an Alternating Current outputvoltage to power an EL lamp. A current limiting circuit is providedwithin the inverter. The method ensures that when the EL lamp is poweredby the inverter, the current limiting circuit limits a maximum peakcurrent received by a user contacting an output of the inverter orcontacting the EL lamp to less than about 0.7 mA.

In accordance with yet another aspect of the invention, a method ofcompensating for decay of an electro-luminescent (EL) lamp provides aninverter including a constant current limiting circuit constructed andarranged to be connected to a direct current input voltage source and toregulate electrical current. The inverter also includes a resonancesource circuit receiving an output voltage from the constant currentsource circuit. The resonance source circuit provides an alternatingcurrent output voltage to the EL lamp. As the EL lamp ages, the outputvoltage from the constant current source circuit is increased, thusincreasing the output voltage from the resonance source circuit, therebycompensating for a decay of the EL lamp.

Other objects, features and characteristics of the present invention, aswell as the methods of operation and the functions of the relatedelements of the structure, the combination of parts and economics ofmanufacture will become more apparent upon consideration of thefollowing detailed description and appended claims with reference to theaccompanying drawings, all of which form a part of this specification.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be better understood from the following detaileddescription of the preferred embodiments thereof, taken in conjunctionwith the accompanying drawing in which:

FIG. 1 is a schematic of a current limiting device for an inverter of anEL lamp, provided in accordance with the principles of the invention.

DESCRIPTION OF THE EXEMPLARY EMBODIMENT

With reference to FIG. 1, a limited current circuit (LCC) device for aninverter of an EL lamp is shown, generally indicated at 10, inaccordance with the principles of the present invention. The LCC device10 includes a constant current source circuit, generally indicated at12, and transformer resonance source circuit, generally indicated at 14.The LCC 10 is part of a conventional inverter 11 for an EL lamp 28. Theinverter 11 converts DC voltage to AC voltage (preferably 50-80 Vrms)and frequency (preferably 250-1000 Hz) for operating the AC powered ELlamp 28.

The constant current source circuit 12 is a linear transistor pass typeregulator. In order to reduce a drop voltage of a transistor 16, a PNPtype transistor is used. The emitter is connected to the inverter inputvoltage source (which is typically 5.0 or 3.3V DC). A current sensor 18is connected with the collector of the transistor 16. The sensor 18 ispreferably in the form of a small resistance shunt (preferably 0.2 ohm)that is also intended to reduce the drop voltage. An operationalamplifier 20 is connected to the shunt resistor 18 to amplify the passcurrent. A reference voltage is established from a resistance divider 21connected to the positive input of another operational amplifier 22. Thenegative input of amplifier 22 is connected to the output of amplifier20. The base of transistor 16 is then connected to the output ofoperational amplifier 22 to complete a constant current feedback loop.

The resonance source circuit 14 includes a transformer 24 having threesections or phases for resonance windings. N1 is connected to the outputof the constant current source circuit 12, current limiting resistor 15and the PNP derive transistor 16. N2 is a step-up winding on N1 which iscoupled to the base circuit through a capacitor 26. N3 is a separatewinding connected to the EL lamp 28 and output ground.

When an EL panel or lamp 28 is provided power and/or is lit, power fromthe constant current source circuit 12 flows to the transformer 24 andfeedback winding N2 forces the derive transistor 16 to switch on andoff. The coupled voltage on the N3 winding will resonate with the ELlamp receiving the AC output. When a user such as a human body (orsimilar resistance) touches the high voltage output of the invertercontaining the LCC 10 (or touches the EL panel or lamp 28), theoscillation is substantially detuned and insufficient energy is then fedthrough the capacitor 26 into the base of transistor 16. The basecurrent of the derive transistor 16 will keep the Direct Current (DC) ata constant value. The constant current source circuit 12 limits thecurrent flow into winding N1 and the derive transistor 16. The currentlimiting resistor 15 has also a voltage drop, and the emitter voltage oftransistor 16 will then drop so that the oscillation stops. Since N3 isa separate winding, the current will not draw from the constant currentsource circuit 12 to the human body (or similar resistance).Experimentally, this approach shows the peak current is significantlybelow UL's current requirements of 0.7 mA and is typically about 0.2 mA.

The LCC 10 can be employed in a circuit or an integrated circuit and/orpartially embedded in a semiconductor chip.

Thus, the LCC 10 can be controlled to levels below the thresholdmandated by UL and enables keyboards and keypads for computer andrelated products to be backlit with EL panels and substantially achieveUL certifications, and Conformité Européenne (CE) certifications forEurope. The LCC 10 can be controlled to levels below the thresholdmandated by UL and enable keyboards and keypads for computer and relatedproducts to be backlit with EL panels and substantially achieve computerrelated safety certifications like Certification Body (CB). Control ofthe LCC 10 provides a much safer backlit environment for EL basedproducts, and control of the LCC 10 enables EL products to be used intoapplications where previously they were unacceptable.

Another feature of LCC is the fact that it is a constant power source tothe derive EL lamp 28. The life as derived by a constant power source ismuch longer than a constant voltage source. As the EL lamp 28 ages, itscapacitance will increase. Since the autotransformer type inverter has aconstant output voltage to the derive EL lamp 28, the current fed intothe resonance circuit 14 and to the EL lamp 28 will decrease, resultingin poor life. In order to maintain constant current flow from thecurrent source 12, the output voltage of the amplifier 22 will increaseand reduce the base current of the pass transistor in the current source12. The enhanced conduction of the pass transistor generates a higheroutput voltage of the pass transistor. The increased input voltage tothe resonance circuit 14 increases the output voltage to EL lamp 28. Thebrightness of the EL lamp depends on the magnitude of applied voltage.Thus, the (brightness) decay of the EL lamp 28 can be compensated toextend the life of the EL lamp 28.

Furthermore, since the EL lamp 28 is powered by the inverter 11,adjusting the setting of the current of the constant current source 12can dim the output of the inverter 11.

The foregoing preferred embodiments have been shown and described forthe purposes of illustrating the structural and functional principles ofthe present invention, as well as illustrating the methods of employingthe preferred embodiments and are subject to change without departingfrom such principles. Therefore, this invention includes allmodifications encompassed within the spirit of the following claims.

1-18. (canceled)
 19. A method of compensating for decay of anelectro-luminescent (EL) lamp, the method including: providing aninverter having a constant current limiting circuit constructed andarranged to be connected to a direct current input voltage source and toregulate electrical current, the inverter also including a resonancesource circuit receiving an output voltage from the constant currentsource circuit, the resonance source circuit providing an alternatingcurrent output voltage to the EL lamp, and as the EL lamp ages,increasing the output voltage from the constant current source circuitand thus the output voltage from the resonance source circuit, therebycompensating for a decay of the EL lamp.
 20. The method of claim 19,further including changing a current setting of the constant currentsource circuit to dim the output voltage of the resonance sourcecircuit.
 21. The method of claim 19, wherein the resonance sourcecircuit includes a transformer with a separate winding electricallyconnected to the EL lamp and output ground,
 22. The method of claim 19,wherein the constant current source circuit includes a transistor, anemitter of the transistor is connected to the input voltage source, acurrent sensor is connected to a collector of the transistor, a firstamplifier is connected with the current sensor to amplify a passcurrent, a resistance divider, establishing a reference voltage, isconnected to a positive input of a second amplifier, a negative input ofthe second amplifier being connected to an output of the firstamplifier, a base of the transistor is connected to an output of thesecond amplifier to complete a constant current feedback loop defined bythe current source circuit.